The invention relates to a method for the photoablation of the cornea with a laser beam, with the aid of which a multiplicity of consecutive partial ablation operations can be undertaken, and to a device with a laser radiation source for carrying out this method. The invention can be used, for one thing, to measure intraocular thicknesses and distances in the front eye section immediately before, during and immediately after surgical operations and treatments of the cornea. A photorefractive treatment can be controlled in real time depending on these measurement results. Control in real time leads to enhanced safety for the patient and to an improved accuracy of the photorefractive eye correction.
U.S. Pat. No. 5,493,109 discloses an ophthalmologic surgical microscope which operates with optical coherence tomography. Here, it was the surface shape of the cornea and the optical refractive power thereof that was determined.
The object of the invention is to create a method and a device with the aid of which the cornea can be safely treated.
In terms of method, the object is achieved by virtue of the fact that the thickness of the cornea, and not only its surface shape and its optical refractive power are determined before and/or after each partial ablation operation with the aid of a measuring device co-operating with a Michelson interferometer. In addition, the position of the region envisaged for the ablation is determined and clearance for ablation is given only if the region is located within prescribed tolerance values. Defective ablation operations such as can occur owing to an eye movement, for example, are thereby excluded.
As a consequence of the determined thickness values of the cornea, the measurement device then guides the laser beam with corrected intensity in a controlled fashion over regions of the cornea that are still to be corrected in terms of thickness. The partial ablations are undertaken thereby in such a way that a prescribed cornea thickness is not undershot in conjunction with compliance of a prescribed cornea profile.
In terms of the device, provision is made for this purpose of a laser radiation source, a control device and a measuring device. The control device can have a beam deflecting unit with the aid of which the laser beam can be guided transversely over the cornea. The device according to the invention further has a measuring device with a Michelson interferometer with a center wavelength of the measuring radiation source in the region of 1,310 nm, and a monomode fiber for this wavelength in the reference arm such that the optical wavelength in air in the measuring arm, which is required for carrying out the measurement, can be compensated acceptably.
As is described below, various calibration curves relating to a determination of the optimum distance of the treatment site as well as inclination tolerances of the latter in relation to the treating laser beam axis are stored in the control device.
In particular, signal-distorting and signal-reducing consequences of the dispersion are minimized by the use of a measuring radiation source in the region of 1,310 nm with a bandwidth of up to approximately +/xe2x88x92100 nm. Specifically, if use is made of a radiation source with an arbitrarily prescribed center wavelength, there is generally a relatively long path in air of the order of magnitude of one meter in the measuring arm as a consequence of the structure prescribed by the operating microscope, the fundus camera or the laser. This path in air must be compensated in the reference arm either by a corresponding path in air or in another medium (preferably in a monomode fiber for space saving reasons), so that the required interference signals can be generated. If the path covered in air in the measuring arm is compensated in the reference arm by a corresponding distance in air, it is true that the dispersion produces only a slight signal distortion and signal reduction. However, in this case the reference arm can no longer be of compact design. If, however, the path covered in air in the measuring arm is compensated by a monomer fiber in the reference arm, relatively high signal distortion and signal reduction normally occur, because the dispersion properties differ in the reference arm and measuring arm. Very surprisingly, however, there is a wavelength region around 1,310 nm where this signal distortion and signal reduction are minimized such that the optical path of a measuring arm in air several meters long can be compensated by a path covered in the reference arm in a monomode fiber, without the interference signal being disturbed thereby. The interferometer can be of exceptionally compact and inexpensive design through the compensation of an optical path in a fiber.
In a preferred embodiment, the laser parameters such as the diameter on the cornea, intensity, power, energy and pulse duration of the laser radiation can also be controlled so that it is possible to take account of environmental influences on the result of the photorefraction such as air humidity and air temperature in the operation room, as well as patient-specific influences such as vaporization of chemicals stored in the operating room [for example (open) alcohol containers, (open) containers for cleaning agents], age, sex, composition of the patient""s cornea and temperature of the patient""s cornea. It would also be possible to take account of patient-specific data.
In order to render this possible, measurements have been made in previous research of the cornea thickness as a function of air humidity, air temperature in the operating room, age, sex, composition of the cornea and temperature of the cornea. The results of these measurements are then used to determine correction factors that feature in the control of the laser beam. An excellent result can then be achieved even given deviations from the normal conditions. It is even possible in the case of patients with irregular astigmatism, keratoconus or an irregular, corrugated cornea surface to undertake photorefractive corrections that reach the limit of approximately 20/5-20/8 (depending on pupil size) set by the laws of optical detraction.
Using the measuring arrangement described here it is possible for the first time to determine the front surface of the cornea, the rear surface of the cornea, the distance of the cornea from a reference site, the inclination of the cornea and thus the co-ordinates of the entire cornea in three-dimensional space online during the entire refractive surgical operation and this, moreover, in a contactless fashion.